1. Field of the Invention
The present invention relates to a method and apparatus for transmitting a Channel State Information Reference Symbol (CSI-RS) and data. More particularly, the present invention relates to a method and apparatus for transmitting the CSI-RS and data with partial muting of the CSI-RS.
2. Description of the Related Art
Mobile communication systems are currently evolving from basic communication devices into high-speed, high-quality wireless packet data communication systems that provide data services and multimedia services beyond the early voice-oriented services. Recently, various mobile communication standards, such as High Speed Downlink Packet Access (HSDPA) and High Speed Uplink Packet Access (HSUPA), both defined by 3rd Generation Partnership Project (3GPP), High Rate Packet Data (HRPD) defined by 3rd Generation Partnership Project-2 (3GPP2), and Institute of Electrical and Electronics Engineers (IEEE) 802.16, have been developed to support the high-speed, high-quality wireless packet data services.
The 3rd generation wireless packet data communication system of the related art, such as HSDPA, HSUPA and HRPD, uses such technologies as an Adaptive Modulation and Coding (AMC) method and a channel-sensitive scheduling method in order to improve transmission efficiency. With the use of the AMC method, a transmitter can adjust the amount of transmission data according to the channel state. For example, when the channel state is not good, the transmitter reduces the amount of transmission data to match a reception error probability to a desired level, and when the channel state is good, the transmitter increases the amount of transmission data to efficiently transmit a large volume of information while matching the reception error probability to the desired level. Using the channel-sensitive scheduling resource management method, the transmitter, since it selectively services a user having a superior channel state among several users, can increase in the system capacity, as compared with a transmitter that allocates a channel to one user and services the user with the allocated channel. Such capacity increase is commonly referred to as a multi-user diversity gain. In brief, the AMC method and the channel-sensitive scheduling method are methods for receiving partial channel state information being fed back from a receiver, and applying an appropriate modulation and coding technique at the most efficient time determined depending on the received partial channel state information.
Recently, intensive research is being conducted to replace Code Division Multiple Access (CDMA), which is the multiple access scheme used the 2nd and 3rd generation mobile communication systems, with Orthogonal Frequency Division Multiple Access (OFDMA) in the next generation mobile communication system. 3GPP and 3GPP2 have started standardization work on evolved systems based on OFDMA.
It is known that OFDMA, compared to CDMA, is expected to increase capacity at least in part due to performing scheduling in the frequency domain (i.e., Frequency Domain Scheduling). While capacity gain can be obtained from the time-varying channel characteristic using the channel-sensitive scheduling method, more capacity gain can be obtained using the frequency-varying channel characteristic.
In order to increase the capacity gain with the aforementioned methods, the information on the status of the radio channel should be acquired. The more accurate the channel status information, the more the capacity gain can increase. In a case of the channel status information measurement using a reference signal, the measurement accuracy is improved as the signal to interference plus noise power ratio of the received reference signal is increased. Accordingly, the interference power to the received reference signal should be reduced in order to improve to the channel status information accuracy. Among various related art methods to reduce the interference to the reference signal, the present disclosure deals with a method to reduce the interference to reference signals transmitted by different transmitters using a muting technique for keeping the time, frequency, antenna, and code resource with which the reference signal of another transmitter is transmitted empty.
FIG. 1 is a diagram illustrating a Channel State Information Reference Symbol (CSI-RS) reuse pattern and muting pattern according to the related art.
Referring to FIG. 1, part (a) shows an exemplary CSI-RS pattern of which a reuse factor is 6 for use in a Long Term Evolution Advanced (LTE-A) system.
In part (a) of FIG. 1, the Resource Elements (REs) numbered from 0 to 7 and having the same background pattern shows a CSI-RS pattern per cell, and 6 different CSI-RS patterns are shown. For one cell, one CSI-RS pattern is used such that 6 adjacent cells can use different CSI-RS patterns.
Part (b) of FIG. 1 shows an exemplary subframe in which a transmitter of one cell transmits the CSI-RS. In a case of part (b) of FIG. 1, the cell uses the CSI-RS pattern 0. Since the positions of the CSI-RS of other cells are used for data transmission, the signal of the cell can affect interference to the CSI-RSs of the 5 adjacent cells using different CSI-RS patterns. Likewise, the 5 adjacent cells affect interference to the CSI-RS of the cell using the pattern of part (b) of FIG. 1. Such interferences degrade channel estimation performance based on the CSI-RS.
Part (c) of FIG. 1 shows an exemplary subframe using the muting technique in which no data is transmitted on the resources that are used for CSI-RS transmission of other cells to mitigate channel estimation performance degradation caused by interference. Since the subframe configuration of part (c) of FIG. 1 can reduce the interference to CSI-RS as compared to the subframe configuration of part (b) of FIG. 1, it is expected to improve the channel estimation performance based on the CSI-RS. However, the subframe configuration of part (c) of FIG. 1 uses a small number of REs for data transmission as compared to the subframe configuration of part (b) of FIG. 1. Accordingly, in order to maintain the same target error rate in both the cases of parts (b) and (c), the subframe of part (c) of FIG. 1 should be configured to transmit the signal at a low Modulation and Coding Scheme (MCS) level as compared to that in the subframe of part (b) of FIG. 1. In a case of part (c) of FIG. 1, since the loss caused by the small number of REs for transmitting data is more affective that the gain obtained by the improvement of the channel estimation performance especially when a large number of transmit antennas are used, the system throughput decreases.
There is therefore a need to develop a novel muting technique different from the related art method for addressing the system throughput degradation caused by the shortage of REs for data transmission without compromising the performance improvement effect caused by the channel estimation performance enhancement with the muting technique, and method and apparatus for transmitting a control signal for supporting the novel muting technique.